This is not something strange. Note that the reinfection is very mild. Doesn't say it's because of the trained immune system or that the new strain was milder.
This is a scare mongering story.
1st infection - 3 days after getting virus, admitted to hospital with no symptoms --> Immune system handled it
2nd infection - Said to have virus based on RT-PCR - not a valid test of disease! - and more obvious the second time, he never showed any indication of being sick
Of course this is 1 patient and there is nothing in the article to even prove what they say happened even happened. It could have been a successful clear of the virus not a reinfection.
Lastly, antibodies are not the entire story on the immune system.
The conclusion for the uneducated is "oh my god its dangerous and this brutal!" The reality is 180 degrees different.
This may be used to push vaccines.